1. Field of the Invention
This invention relates to an improved method and apparatus for providing a desired hydraulic clamping pressure in an hydraulically operated clamping press. More particularly, the present invention relates to a method and apparatus for controlling the clamping force exerted by the mold clamping system of an injection molding machine in order to provide a mold closing force that corresponds with a predetermined mold closing force, so that plastic material injected into the mold under high pressure does not flash from the mold as a result of mold separation during the injection step.
2. Background of the Related Art
Injection molding machines include a plastication and injection unit for converting solid polymeric material from solid pellets to a flowable liquid that can be injected under pressure into a closed mold cavity in order to completely fill the mold cavity and provide the desired molded article. The mold in which the desired article is formed is typically a multiple part mold, which could be as few as two parts, that is adapted to be closed while the polymeric material is being injected into the cavity defined by the mold parts when they are in contact with each other, and which is then adapted to be opened, by separating the mold parts, in order to permit removal of the molded article.
Such machines include apparatus for moving the mold parts into and out of engagement with each other, and also include a clamping arrangement that is intended to apply a clamping force to hold the molds in tightly engaged relationship while the injection of the polymeric material is taking place. In that regard, injection pressures can typically range in the thousands of pounds per square inch, and when that pressure is exerted within the closed mold cavity, the pressure acts on the mold cavity surfaces with a resultant force that tends to urge the mold parts apart from each other. Consequently, clamping apparatus is provided for holding the mold parts tightly together to prevent such separation, and typically the clamping apparatus includes an hydraulically operated clamp that is capable of providing high clamping forces on the mold halves, the magnitude of the forces depending upon the sizes of the mold cavities and on the areas of the surfaces over which the pressure of the injected plastic material acts. Injection molding machines are presently commercially available that are capable of providing clamping forces as large as 1,000 tons and more.
In the past, the prevalent practice that was followed to maintain the clamping force during the time the polymeric material was being injected into the mold cavity, and during the time the injected material was being cooled while within the mold, involved the provision of at least one hydraulic pump for the clamp cylinder, and the pump was operated continuously during the time clamping forces were desired in order to maintain the hydraulic pressure in the clamping system at a sufficient level to provide the desired clamping force. The maximum hydraulic pressure in the clamping system was controlled by providing a relief valve between the pump and the hydraulic clamp cylinder, so that the hydraulic pressure within the clamp cylinder did not exceed a predetermined value. Such a system is disclosed in U.S. Pat. No. 3,825,386, which issued July 23, 1974, to Wolfert Bello et al., in which hydraulic pressure from a pump is maintained continuously on a pressure cylinder to maintain a desired clamping force, and a pressure controlled directional valve is provided to regulate the volume of flow from the continuously operating pump to the clamp cylinder. Such previously favored arrangements for maintaining clamping force involve additional equipment and cost in terms of requiring a pump merely for the clamp force maintenance function, and they also require the expenditure of unnecessary energy by continuously driving the pump while simultaneously conveying a part of the pump outlet through a pressure relief valve to a fluid reservoir, which resulted in wasted energy.
It is an object of the present invention to overcome the deficiencies of the previous clamp force maintenance arrangements.
It is a further object of the present invention to provide a clamp force maintenance system in which the need for a continuously operating hydraulic pump is eliminated.
It is a still further object of the present invention to provide a clamp force maintenance system that does not require either a continuously operating hydraulic pump or a pressure relief valve in the clamp force maintenance circuit.